1. Field of the Invention
The present invention relates to a polyester resin composition and specifically to a polyester resin composition prepared by highly dispersing a layered silicate and excellent in strength, heat resistance, barrier properties, and the like.
2. Description of the Related Art
Thermoplastic polyester resins represented by polyethylene terephthalate (PET), polybutylene terephthalate (PBT), or polyethylene naphthalate (PEN) have excellent mechanical strength, heat resistance, and molding processability and therefore are used for a wide variety of purposes such as moldings, films, and fibers.
A variety of resin compositions are known in which reinforcing fibers such as glass fiber and carbon fiber or inorganic fillers such as calcium carbonate, clay minerals, and mica are incorporated in these thermoplastic polyester resins and, then kneaded to be reinforced from the viewpoint of promoting a further improvement in the mechanical strength (mechanical characteristics) and heat resistance. However, a problem introduced by these inorganic materials is that a sufficient improvement in the mechanical strength and heat resistance cannot be achieved since the materials are difficult to form finely divided particles and disperse uniformly by simple mixing and kneading. For example, JP-A No.6-56975 discloses a resin composition prepared by dispersing kaolin or talc into a PET-PEN copolymer resin, and this resin composition also encounters a similar problem. Particularly, when resistance to thermal decomposition is not sufficient, the polyester itself undergoes hydrolysis during high-temperature processing and fails to give substantially moldings or films having good appearance and physical properties. An attempt to add a large amount of the inorganic materials in order to improve further the mechanical strength and heat resistance results in an increase in specific gravity and deterioration in processability.
In recent years as a technique for solving the above problem, a variety of composite materials have been proposed which are improved in the heat resistance and mechanical characteristics by dispersing a small amount of a layered silicate on a molecular level in a polyester resin. For example, JP-A No.62-74957 discloses a composite material prepared by dispersing uniformly a sheet clay mineral into a polyamide and having excellent strength, stiffness, and heat resistance. Use of a small amount of the sheet clay mineral surely enables a significant improvement in the mechanical strength and heat resistance, but in the case of the polyester resin, a composite material in which the sheet clay mineral is uniformly dispersed cannot be obtained like the case of the polyamide by the method disclosed in the above document. With relation to this technique, a further technique for improving dispersibility of the sheet clay mineral with the aid of a compatibilizing agent is also disclosed. However, only moldings which are small in the effects of improving the mechanical strength and heat resistance and large in deterioration in toughness are obtained (For example, see JP-A No.3-62846).
Furthermore, for example, JP-A No.8-53572 discloses a resin composition in which a layered silicate having an unreacted compound among the sheets is dispersed in a polymer. However, when a polyester resin is used as a matrix resin, the polyester resin is subject to hydrolysis because the resin is exposed to high temperatures during molding processing and cannot give moldings and films having good appearance and physical properties.
Furthermore, for example, JP-A No. 11-130951 discloses a polyester composite material containing a layered silicate organized with a phosphonium salt, stating that the mechanical strength and heat resistance can be improved. The composite material can surely be prevented from coloring stemming from the thermal decomposition of the organizing agent itself to maintain a good color tone and simultaneously can be somewhat improved in the mechanical strength and heat resistance. However, since the presence of an organic cation of the organizing agent promotes the hydrolysis of the polyester resin at high temperatures (in molding processing), the mechanical strength and heat resistance of the composite material is deteriorated, failing to give moldings and films practically usable.
This document discloses that the layered silicate can be highly dispersed in the polyester resin by allowing an ionic bond of an organic phosphonium ion to the sheets to give moldings excellent in strength, stiffness, and heat resistance. However, it is impossible to prevent the hydrolysis at high temperatures as described above (For example, see JP-A No.11-1605). Moreover, the compositions and resin films also have a problem in hydrolysis resistance at high temperatures (For example, see JP-A Nos.2000-53847 and 2000-327805).
In addition, techniques for promoting the mechanical properties and heat resistance and improving a balance among physical properties of the polyester resin composition are proposed (For example, see JP-A Nos.2001-323143, 2001-323144, 2001-323145, and 2001-329150). However, all the techniques need complicated steps or a step of treating the surface of a sheet compound with a polyether compound to encounter a problem of causing the productivity to deteriorate.
Furthermore, for example, JP-A No.2000-86232 discloses a polyester resin composition containing a layered silicate in which an aromatic dicarboxylic acid residue having a metal sulfonate group as a substituent is copolymerized in a ratio of 0.01 to 10 mol % of the dicarboxylic acid residue. This reference states that the layered silicate is highly dispersed without losing transparency and gas barrier properties and mechanical strength are improved. However, the document limits the polyester composition to those copolymerized with the aromatic dicaroxylic acid monomer having 5 mol % or less of a metal sulfonate group and moreover does not mention anything as to the necessity of antioxidant.
JP-A No.2002-88236 discloses a polyester composite material in which a polyester copolymer containing 10 mol % or less of an aromatic dicarboxylic acid which has a sulfonic acid group as an acid component is allowed to exist. In a thermoplastic aromatic polyester, a swellable layered silicate is uniformly dispersed at a molecular level. However, this document does not mention the antioxidant and does not disclose a necessary molar ratio of the aromatic dicarboxylic acid component having a sulfonic acid group in the total polyesters of the polyester composite material, either.